Fix MSP spouse deeming and resource scoping

[daphnehanse11]

Summary

• apply spouse deeming in MSP countable income and compare married applicants using spouse-aware income
• scope MSP resources to the applicant and spouse instead of the full SPM unit, and align MSP eligibility with the resolved MSP category
• tighten MSP category boundary tests and backfill Medicare premium history used by MSP benefit coverage

Testing

• uv run --extra dev ruff format .
• uv run --extra dev ruff check policyengine_us/variables/gov/hhs/medicare/savings_programs policyengine_us/parameters/gov/hhs/medicare/savings_programs policyengine_us/tests/policy/baseline/gov/hhs/medicare/savings_programs
• uv run policyengine-core test policyengine_us/tests/policy/baseline/gov/hhs/medicare/savings_programs -c policyengine_us
• uv run --extra dev ruff check policyengine_us/parameters/gov/hhs/medicare policyengine_us/tests/policy/baseline/gov/hhs/medicare changelog.d/codex-msp-ssi-deeming-assets.fixed.md
• uv run policyengine-core test policyengine_us/tests/policy/baseline/gov/hhs/medicare/eligibility/base_part_a_premium.yaml policyengine_us/tests/policy/baseline/gov/hhs/medicare/eligibility/base_part_b_premium.yaml policyengine_us/tests/policy/baseline/gov/hhs/medicare/savings_programs/msp_benefit_value.yaml -c policyengine_us